1. Field of the Invention
The present invention relates to a drive unit for moving parts of articles of furniture.
2. The Prior Art
Actuators driven by a reversible low volt DC motor, typically 24V, are used for adjustable articles of furniture, such as beds, chair and tables. In a linear actuator, the motor, via a transmission, drives a spindle having a nut fixed against rotation so that the nut can move to and fro on the spindle depending on the direction of rotation of the spindle. The nut has arranged thereon a tubular rod whose free end is provided with an attachment for securing in the structure. Typically, the transmission simply consists of a worm gear, where the worm is provided in an extension of the motor shaft and the worm wheel secured directly on the spindle. An example is disclosed in WO 02/29284 to Linak A/S. A special structure of linear actuators is called lifting columns, e.g. intended for table legs. Rotary actuators have so far not been as common as linear actuators in connection with furniture, but a single example based on a specially constructed planetary gear is described in WO 01/17401 to Linak A/S. Also known are other forms of drive units constructed specifically for use in furniture. In the furniture business, the price of the actuators is a decisive factor, which has become particularly pronounced recently. This, of course, has resulted in a development toward simpler structures. Thus, it may be mentioned that for cost-reducing purposes the power supply is dimensioned in view of the circumstance that it is loaded only briefly when the article of furniture is adjusted. Or put differently, it is allowed to overload the power supply briefly. For example, in case of a height-adjustable table which may be adjusted from 650 mm to 1250 in the height, it takes 12 seconds to get from one extreme position to the other at a speed of 50 mm/sec. To avoid continuous overload operation, running is just allowed for a given interval of time so that the power supply is not superheated.
Especially in the furniture business, a decisive aspect is the level of noise, and with a demand for less expensive actuators it is even more difficult to meet the requirement for low-level noise actuators.
During adjustment of the article of furniture mechanical noise occurs, such as transmission noise, noise in the suspension of the adjustable element, noise because of vibrations that propagate in the structure, etc. When the most powerful single sources of noise have been remedied, it is a complex noise picture that emerges, it being very difficult, if not impossible, to locate the individual sources of noise.
In case of furniture, the level of noise is a significant factor in the sense that this must be as low as possible. This applies to beds, including hospital beds and sickbeds, as well as armchairs and tables, including desks. In the furniture structure and in the drive unit, various common noise-reducing measures may be taken, such as the use of rubber/plastics suspensions, plastics bushings, lubricants, etc. to counteract the noise, but in spite of this it is still desirable to reduce the level of noise additionally.
Some of the noise originates from the motor, where part of the noise is caused by the structure of the rotor with axis-parallel air gaps between the iron flanges on which the coils are wound. This gives small, but noticeable discontinuities in the magnetic field that cause vibrations which propagate in the structure and cause acoustic noise. To obviate the discontinuity, it has been proposed to twist the armature so that the air gap is not axis-parallel. The phenomenon is described in another connection in U.S. Pat. No. 4,616,151.
Another part of the noise from the motor is caused by an axial movement of the rotor, which likewise results in vibrations and thereby acoustic noise. Owing to manufacturing tolerances it is difficult to do anything about the problem. The phenomenon is also described in another connection inter alia in U.S. Pat. No. 5,497,039 and in U.S. Pat. No. 6,069,422, both of which propose solutions for fixing the rotor in the axial direction via the magnetic fields of the stator.
Another aspect of the acoustic noise is the transmission noise, part of which originates from the engagement between the gear wheels or more particularly the varying degree of the engagement. In this context, reference may be made to WO 01/94732 A1 to VKR Holding A/S, which addresses noise problems in relation to window openers, focus being on noise caused by clearance between the components of the transmission, it being attempted to reduce the noise by controlled engagement between a worm and a worm wheel.
The outlined solutions are expensive and complicated or cannot readily be implemented in the present connection.
The invention is based on an ever increasing wish for reducing the noise level of linear actuators, bearing in mind that this must not lead to increased or appreciably increased costs of these.